Project 280+P (Part I)

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Introduction: This is a basic build up of a Mercury 2.5L 280 horsepower engine for a lake use STV River Rocket. This build up is based on real world requirements and expectations for use on pump fuels. It is not a race engine, but it may be used for occasional informal racing in a heads up style.

As a firearms enthusiast, I chose the name of the project based on my relationship with reloading cartridges. A cartridge load that is significantly more powerful than normal over the counter loads is termed +P for added power. This term seemed appropriate for this project.

This is a basic performance build, not a racing project. More power can be made, and more efficiency can be gained. But this project takes an approach that is based on goals for how I use my STV and what I wanted THIS engine to achieve while being easy to reproduce by others. In many places I have assumed the reader has a given understanding of these engines, how they operate, and basic performance and engine terms. If something is not clear, PLEASE ask questions, and I will answer as best I can. I am not a professional engine builder. I am not an outboard GURU. I am an enthusiast with a good amount of experience racing and running performance outboard powered boats. This project reflects these points and many other enthusiasts should find it informative and interesting.

I do not expect everyone to need or want the limitations and goals of this engine project. I dont even expect everyone to agree with the way this engine is built. This is simply the way this engine was built. Take it as it is a fun project.

Background: The Mercury 280 engines became available in 1999 as a replacement for the 260 hp engines previously available to consumers through the Mercury Racing product line of outboards. These engines have proven to be reliable while offering good performance in an outboard marine engine. Many of these engines now have a large amount of time and use on them. As a result, many are in need of rebuilding and many of their owners would probably like to see what is involved in getting more from their engines in place of a simple rebuild.

Up until the release of the Drag engines in mid 1995 (as 1996 models), the 260 was the most powerful engine available in the Mercury Racing line of outboard engines. It was a hand ground block in which nearly each year the port specs changed, and the offshore race engines tended to be the best of the bunch. The Drag block is a new design and the 280 engines are based on this engine, with 150-160 psi compression using o-ring heads (a bit more than the 260 engines), and slightly lower port timing than the best 260 engines and significantly less than the Drag engines.

Situation: The 280 engines make peak power at around 8000-8200 rpm with around 286-289 crankshaft HP. [This compares reasonably with the 260 engines, but the power is slightly increased and broader.] Compare this to the Drag engines with more power (335-ish hp) at 8400-8600 rpm. The late model Drag engine uses a single ring piston for less drag (about 5 hp at peak) but more blow-by, resulting in less power below 6500 rpm, and a reduced ring life, compared to the dual rings per piston on the 280 engines.

The block is nearly the same as the Drag block, and is cast in the same molds. The major difference is the port timing and the roof of the exhaust ports. The exhaust port roof has a longer roof that turns downward in the exhaust chest. This longer, down turned, roof is said to be worth 5 ft.-lb. of torque at low rpm, but its length intrudes into the port resulting in a restriction that blocks exhaust gas flow and hinders rpm in the over-rev range (the part of the rpm band above peak power rpm). The 260 exhaust chest is shaped differently, with the exhaust ports being shaped more like tubes than the 280 and Drag/S3000 engines, and the slots on each side of the chest near the short side of the ports.

The 280s tend to have an exhaust port height of 1.50-1.52 measured from the top of the block deck to the top of the port radius and 1.605-1.603 to the top of the port generally the same as the 260 engines. The deck height on the 280 and 260 is also about .004 inches higher than the deck on the Drag engine. The top of the entry to the transfer and boost ports is generally 4.575 inches down from the deck, which is the same as the Drag engines, but taller than the 260s at 4.751 inches measured from the deck, and gives a larger port opening than the 260 engines. The boost ports are generally set at around 2.145 inches (at the top of the radius), while the transfer ports are set at 2.110. This compares to the good 260s with boost ports at 2.135 and transfers at 2.112, and the early Drag engines with boost height at 2.110 on the outside ports and 2.065 for the center boost port, and 2.060 on the transfer ports with an exhaust port height of 1.490 at the top of the radius and 1.540 at the port roof.

Realize that many of these engines will vary with their port timing (especially the hand ground Drag and OSR 260 engines) and many people measure port height differently. But basically, all of this means that the 280 engines have port heights that are several degrees lower (in relation to crankshaft rotation) than the Drag engines. And that the peak power will be delivered at a reduced rpm as a result of this.

Mission: Since this is a lake engine, and lake boats tend to be heavier and carry heavier loads, building this engine the same as I would a race engine will defeat the intent of this project. First of all, a lake engine should run well on pump gas or pump gas with an octane boosting additive. Second, it should be at least as reliable as the stock engine. And third, it should produce better power over a broader range than stock. If this sounds like wanting our cake and eating it too, then follow along with the way we intend to accomplish this project.

I am not a fan of the stock 280 ECU and ignition system, so for this project, 260 electronics and an A6 ECU will be used. This will allow the engine to be turned much higher for occasional drag racingshould the need present itself.

Power vs- Torque: Realize that horsepower is a calculation of torque at a given rpm [HP=TQ X RPM / 5252]. So to make more power you can simply move the rpm band of the engine higher with more port timing and you have more power at a higher rpm. But you have just robbed the low end rpm of very much needed torque in order to make more power. This is the same as putting a camshaft with more duration into a truck engine, and the results are nearly the same. Personally, I think this is lazy, and usually results in a lazy engine for lake use. There are better ways to make more power in the same rpm range and even extend this range by a couple of hundred rpm on both the higher and lower ends of the power band; it just takes more work to do it.

If the ports are made more efficient, they can provide flow for a couple of hundred rpm than the stock ports without losing low rpm velocity, with reasonable port timing that is better suited to the octane limits of pump gas or pump fuels with a small amount of octane booster. This efficient flow can provide a broad torque band, and the added flow allows the same torque to be produced for a couple of hundred more rpm, which will give a nice improvement to HP.

A lake boat is a heavily loaded vehicle without a transmission. As a result, it needs a broad power band and lots of low end grunt to get it moving. Without the ability to shift into a higher gear, the engine needs to be able to rev well beyond its peak rpm point is you desire high speed.

For this project I intend the engine to make good power over a broad range and to match all aspects of the engine details to get the most from the engine within this range while keeping most of the build up realistic and cost conscious. The only deviations will be to use some left over parts from my racing program, and these will be noted.

Goals: Since we know that the 280 engines actually produce around 286 hp at 8200 rpm, we need to set a goal for what we want the engine to produce at the end of this project. Based on experience, adding 25% more power is a reasonable amount of an increase within the limits of our project, but should prove difficult to reach. This would result in 357.5 hp. Can this be achieved realistically within our limits? Stick around and lets find out! We might as well set our goals high!

--------------------- Something to think about: --------------

From the above formula for horsepower, we can move the variables around a bit and figure torque. TQ = HP X 5252 / RPM. If we have a peak power of 286 hp at 8200 rpm, the above formula gives TQ = 286 X 5252 = 1502072 / 8200 = 183.1795 ft-lb of torque at 8200 rpm. [This is not peak torque; it is only the amount of torque at 8200 rpm.]

If this same amount of torque is made at 8400 rpm, power is increased. TQ = 183.1795 X 8400 rpm = 15387078 / 5252 = 292.9756 hp.

Realize that torque is what accelerates the boat. And that by moving the peak power to a higher rpm, a slightly lower prop pitch of higher gearing can be used to multiply this torque for better acceleration, but engine life and reliability and low speed flexibility will be reduced. A wise man once said RPM stands for Ruins Peoples Motors something to think about on a lake boat.

If low speed torque is not lost (or even improved), and the additional high rpm power is there when you want it, without a loss in low rpm flexibility a win-win situation is yours for the taking.

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Between business, the SC project, helping get my brother's first boat ready for the magazines, and Honey-do's, my boats are WAY down on the priority list. I have also been porting heads and throttle bodies and building a custom header for my bike (welding merge collectors and exhaust splitters - lots of fun...NOT), now that I have the ability to tune the Keihin ECU for fuel and timing.

The "280+P" engine is almost finished - it is back from replating (a bit of clean up porting on the port entries is left to do), and then I need to assemble it....when I get some time.

I will shoot the remaining photos, and add the next part ASAP. INCLUDING the port map of the finished engine IN DEGREES - for Galen. I normally don't give that out for outboard engines because I get stupid looks when I talk about port timing in degrees for outboard engines. ALL other two-stoke engine builders use timing in degrees (and area in square mm)....for some reason many outboard engine builders seem mystified by a timing wheel.